Offer to submit its voluntary carbon emissions target to a binding resolution would allow UN to verify China's pledges.

China has offered for the first time to submit its voluntary carbon emissions target to a binding UN resolution that would include outside verification that it is sticking to its pledges to curb emissions.

China's target would still be voluntary - a distinction from developed nation targets under Kyoto, Xie Zhenhua, China's chief negotiator, said.

"China is willing to share with the world and we have a willingness to take an open and transparent attitude," he said.

"There is more consensus and our differences are being reduced," he said of the meeting.

"At the end, there will maybe not be a satisfactory deal for everyone but an acceptable one."

On Tuesday top negotiators from more than 190 countries launch the main phase of the two-week summit in Cancun, which comes a year after the much-criticised Copenhagen conference that produced a general agreement to work together, but nothing binding.

Binding commitment

The talks looked set to continue along some of the most divisive issues to future meetings, most crucially, by how much countries will cut carbon emissions blamed for global warming.

"China is willing to share with the world and we have a willingness to take an open and transparent attitude."

Xie Zhenhua, Chinese climate negotiator

With China now the world's top polluter, the United States considers tough, binding and verifiable commitments by the Asian giant to be the best way to sell any climate agreement in Washington.

But last month's election victory by the Republican Party, which includes skeptics of the science behind climate change, all but ended the prospect of the United States approving legislation to restrict carbon emissions.

On a brief visit to Cancun, Steven Chu, the US energy secretary, said Barack Obama, the US president, was committed to meeting US pledges made in Copenhagen to curb carbon emissions by 17 percent by 2020 from 2005 levels.

"He absolutely feels that moving toward a clean energy economy is really about our energy security and about our financial security," Chu said about the president. "It's about our economy; it's about the future of the planet."

However, Chu's low-key visit contrasts with the major push last year at Copenhagen, where Obama and other leaders personally hammered out the final deal.

Even if major emitters meet their stated goals, scientists say it is not enough to achieve Copenhagen's ambition of checking global warming at two degrees Celsius above pre-industrial levels.

The Cancun conference is considering strengthening the target to 1.5 degrees as scientists warn that climate change is already wreaking havoc, with rising sea levels on course to threaten low-lying cities around the world.

"We want countries to recognise that there is a problem and that their figures are not going to help us deal with climate change," he said.

With few expecting a full-fledged treaty anytime soon, the European Union has led calls to extend the Kyoto Protocol past the end of 2012, when requirements under the landmark treaty are set to expire.

"We want countries to recognise that there is a problem and that their figures are not going to help us deal with climate change."

Wendel Trio,Greenpeace

The EU position has triggered protests from Japan. It says Kyoto is unfair by not involving the two top polluters - China, which has no requirements as a developing country, and the US, which rejected the treaty in 2001.

Hoping to break the deadlock, host Mexico paired up developing and developed nations to sort out differences. Britain and Brazil were tasked with looking into the future of the Kyoto Protocol.

Sergio Serra, Brazil's negotiator, feared that Japan's firm position would "have a strong, negative impact on everything that can be negotiated here at the conference".

He hoped to reach a solution under which "if Japan can't completely change its position, at least it cannot be an obstacle to the conference's end result".

Outside the conference, Mexican authorities were stepping up security as busloads of activists and peasants plan to protest on Tuesday against proposals to put the World Bank or markets in charge of climate assistance.

"What we're hoping to achieve is to influence the process. So far it's business as usual - they're trying to make better business," Paul Nicholson of farm workers activist movement Via Campesina, said.